1. Field of the Invention
The present invention relates to an Fexe2x80x94Bxe2x80x94R based permanent magnet having an excellent corrosion-resistant film, and a process for producing the same. More particularly, the present invention relates to an Fexe2x80x94Bxe2x80x94R based permanent magnet which has, on its surface, an excellent corrosion-resistant film having an excellent adhesion to the surface of the magnet; which has a thermal shock resistance enough to resist even a heat cycle for a long period of time in a temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C.; which can exhibit a stable high magnetic characteristic that cannot be deteriorated even if the magnet is left to stand under high-temperature and high-humidity conditions of a temperature of 80xc2x0 C. and a relative humidity of 90%; and in which the film is free from hexa-valent chromium, and to a process for producing the same.
2. Description of the Related Art
An Fexe2x80x94Bxe2x80x94R based permanent magnet, of which an Fexe2x80x94Bxe2x80x94Nd based permanent magnet is representative, is practically used in various applications, because it is produced of an inexpensive material rich in natural resources and has a high magnetic characteristic.
However, the Fexe2x80x94Bxe2x80x94R based permanent magnet is liable to be corroded by oxidation in the atmosphere, because it contains highly reactive R and Fe. When the Fexe2x80x94Bxe2x80x94R based permanent magnet is used without being subjected to any treatment, the corrosion of the magnet is advanced from its surface due to the presence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the degradation and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is assembled into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
There is a already proposed magnet which has a corrosion-resistant metal-plated film on its surface, which is formed by a wet plating process such as an electroless plating process and an electroplating process in order to improve the corrosion resistance of the Fexe2x80x94Bxe2x80x94R based permanent magnet with the above-described point in view (see Japanese Patent Publication No. 3-74012). In this process, however, a acidic or alkaline solution used in a pretreatment prior to the plating treatment may remain in pores in the magnet, whereby the magnet may be corroded with the passage of time in some cases. In addition, the magnet is poor in resistance to chemicals and for this reason, the surface of the magnet may be corroded during the plating treatment. Further, even if the metal-plated film is formed on the surface of the magnet, as described above, if the magnet is subjected to a corrosion test under conditions of a temperature of 60xc2x0 C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 100 hours.
There is also a conventionally proposed process in which a corrosion-resistant film such as a phosphate film or a chromate film is formed on the surface of an Fexe2x80x94Bxe2x80x94R based permanent magnet (see Japanese Patent Publication No. 4-22008). The film formed in this process is excellent in adhesion to the surface of the magnet, but if it is subjected to a corrosion test under conditions of a temperature of 60xc2x0 C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 300 hours.
In a process conventionally proposed in order to improve the corrosion resistance of the Fexe2x80x94Bxe2x80x94R based permanent magnet, i.e., in a so-called aluminum-chromate treating process (see Japanese Patent Publication No. 6-66173), a chromate treatment is carried out after formation of an aluminum film by a vapor deposition process. This process remarkably improves the corrosion resistance of the magnet. However, the chromate treatment used in this process uses hexa-valent chromium which is undesirable for the environment and for this reason, a waste-liquid treating process is complicated. It is feared that a film formed in this process influences a human body during handling of the magnet, because it contains just a small amount of hexa-valent chromium.
On the other hand, in recent years, the field of application of the Fexe2x80x94Bxe2x80x94R based permanent magnet is not limited to the electric industry and the domestic electric appliance industry, and it has been expected that the Fexe2x80x94Bxe2x80x94R based permanent magnet can be applied to fields where it is used in a hard condition. In correspondence to this fact, it is regarded as important that the Fexe2x80x94Bxe2x80x94R based permanent magnet has required characteristics including not only an excellent corrosion resistance under given conditions, but also an excellent thermal shock resistance relative to a variation in temperature. For example, a magnet assembled into parts such as a motor for an automobile must resist a large variation in temperature. To meet such demand, a corrosion-resistant film itself formed on the magnet must be prevented from being cracked or peeled off due to a variation in temperature.
Accordingly, it is an object of the present invention to provide an Fexe2x80x94Bxe2x80x94R based permanent magnet which has, on its surface, an excellent corrosion-resistant film having an excellent adhesion to the surface of the magnet; which has a thermal shock resistance enough to resist even a heat cycle for a long period of time in a temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C.; which can exhibit a stable high magnetic characteristic that cannot be deteriorated even if the magnet is left to stand under high-temperature and high-humidity conditions of a temperature of 80xc2x0 C. and a relative humidity of 90%; and in which the film is free from hexa-valent chromium, and to a process for producing the same.
The present inventors, in a course of various zealous studies made with the above points in view, have paid their intention to the fact that a metal film is formed on the surface of an Fexe2x80x94Bxe2x80x94R based permanent magnet, and a metal oxide film less influencing the human body and the environment is formed on the metal film. A process for forming a primary coat layer on the surface of an Fexe2x80x94Bxe2x80x94R based permanent magnet using a metal as a main component, and forming a glass layer on the surface of the primary coat layer has been already proposed (see Japanese Patent Application Laid-open No. 1-165105). Japanese Patent Application Laid-open No. 1-165105 describes that it is difficult to form a glass layer uniformly, when the glass layer has a thickness of less than 1 xcexcm. However, as a result of further studies made by the present inventors, to be surprised, it has been found that if the metal film is formed on the surface of the Fexe2x80x94Bxe2x80x94R based permanent magnet, and the metal oxide film having a thickness of 1 xcexcm or less is formed on the metal film, the metal oxide film is firmly closely adhered to the metal film on the magnet to exhibit an excellent effect not only to the corrosion resistance under given conditions, but also to a thermal shock resistance with respect to a variation in temperature.
The present invention has been accomplished based on such knowledge. To achieve the above object, according to a first aspect and feature of the present invention, there is provided an Fexe2x80x94Bxe2x80x94R based permanent magnet having a metal oxide film having a thickness of 0.01 xcexcm to 1 xcexcm on the surface thereof with a metal film interposed therebetween.
According to a second aspect and feature of the present invention, in addition to the first feature, the metal film is formed of at least one metal component selected from the group consisting of Al, Sn, Zn, Cu, Fe, Ni, Co and Ti.
According to a third aspect and feature of the present invention, in addition to the first feature, the metal film has a thickness in a range of 0.01 xcexcm to 50 xcexcm.
According to a fourth aspect and feature of the present invention, in addition to the first feature, the metal oxide film is formed of at least one metal oxide component selected from the group consisting of Al oxide, Si oxide, Zr oxide and Ti oxide.
According to a fifth aspect and feature of the present invention, in addition to the first feature, the metal oxide film is formed of a metal oxide component including the same metal component as the metal component of the metal film.
According to a sixth aspect and feature of the present invention, in addition to the first feature, the thickness of the metal oxide film is in a range of 0.05 xcexcm to 0.5 xcexcm.
According to a seventh aspect and feature of the present invention, in addition to the first feature, the content of carbon (C) contained in the metal oxide film is in a range of 50 ppm to 1,000 ppm.
According to an eighth aspect and feature of the present invention, in addition to the first feature, the metal oxide film is formed of a metal oxide essentially comprising an amorphous phase.
According to a ninth aspect and feature of the present invention, there is provided a process for producing an Fexe2x80x94Bxe2x80x94R based permanent magnet, comprising the steps forming a metal film on the surface of an Fexe2x80x94Bxe2x80x94R based permanent magnet by a vapor deposition process, applying a sol solution produced by the hydrolytic reaction and the polymerizing reaction of a metal compound which is a starting material for a metal oxide film, to the surface of the metal film, and subjecting the applied sol solution to a heat treatment to form a metal oxide film having a thickness in a range of 0.01 xcexcm to 1 xcexcm.
According to the present invention, the Fexe2x80x94Bxe2x80x94R based permanent magnet having, on its surface, the metal oxide film having a thickness in the range of 0.01 xcexcm to 1 xcexcm with the metal film interposed therebetween is left to stand under high-temperature and high-humidity of a temperature of 80xc2x0 C. and a relative humidity of 90% for a long period of time, the magnetic characteristic and the appearance thereof are little degraded. In addition, the Fexe2x80x94Bxe2x80x94R based permanent magnet has an excellent thermal shock resistance enough to resist a heat cycle for a long period of time in a temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C.
At least one selected from the group consisting of, for example, Al, Sn, Zn, Cu, Fe, Ni, Co and Ti is used as a metal component for the metal film formed on the Fexe2x80x94Bxe2x80x94R based permanent magnet.
The method for forming a metal film on the surface of a magnet is particularly not limited, but a vapor deposition process is desirable in view of the fact that the magnet and the metal film are liable to be oxidized and corroded.
The vapor deposition process, which may be used, include known methods such as a vacuum evaporation process, an ion sputtering process, an ion plating process and the like. The formation of the metal film may be carried out under common conditions in each of the methods, but from the viewpoints of the denseness of the metal film, the uniformity of the thickness, the deposition rate and the like, it is desirable to employ a vacuum evaporation process or an ion plating process. Of course, the surface of the magnet may be subjected to a known cleaning treatment such as a washing, a degreasing and a sputtering prior to the formation of the film.
It is desirable that the temperature of the magnet during the formation of the metal film is set in a range of 200xc2x0 C. to 500xc2x0 C. If the temperature is lower than 200xc2x0 C., there is a possibility that a film having an excellent adhesion to the surface of the magnet is not formed. If the temperature exceeds 500xc2x0 C., there is a possibility that cracks are generated in the film in a cooling course after formation of the film, whereby the film is peeled off from the magnet.
The thickness of the metal film formed by the above-described process is desirable to be in a range of 0.01 xcexcm to 50 xcexcm, more preferably, in a range of 0.05 xcexcm to 25 xcexcm. This is because if the thickness is smaller than 0.01 xcexcm, there is a possibility that an excellent corrosion resistance cannot be exhibited, and if the thickness exceeds 50 xcexcm, there is a possibility that an increase in manufacture cost is brought about, but also there is a possibility that the effective volume of the magnet is decreased.
The adhesion between the surface of the magnet and the metal film can be enhanced by subjecting the metal film formed on the surface of the magnet by the above-described process to a heat treatment. The heat treatment may be carried out at this time point, but a similar effect can be obtained even by a heat treatment for forming a metal oxide film which will be described hereinafter. It is desirable that the temperature for the heat treatment is equal to or lower than 500xc2x0 C., because if the temperature exceeds 500xc2x0 C., there is a possibility that the degradation of the magnetic characteristic of the magnet is brought about, and there is a possibility that the metal film is molten.
The method for forming a metal oxide film is particularly not limited, but a sol-gel process is desirable in respect of the fact that a metal oxide film can be formed simply and safely, which process comprises the steps of applying a sol solution produced by the hydrolytic reaction and the polymerizing reaction of a metal compound which is a starting material for the metal oxide film, and subjecting the applied sol solution to a heat treatment to form a metal oxide film.
The metal oxide film may be a film formed of a single metal oxide component, or a composite film formed of a plurality of metal oxide components. The metal oxide component may be, for example, at least one selected from the group consisting of aluminum (Al) oxide, silicon (Si) oxide, zirconium (Zn) oxide and titanium (Ti) oxide.
Among the films formed of the single metal oxide, the silicon oxide film (Siox film: 0 less than xxe2x89xa62) can be formed at a low temperature, as compared with a case where a film of another metal oxide component, because the sol solution for forming the film is stable, as compared with a sol solution for forming another metal oxide film and hence, this silicon oxide film is advantageous in respect of that the influence to the magnetic characteristic of the magnet can be reduced. The zirconium oxide film (ZrOx film: 0 less than xxe2x89xa62) is advantageous in respect of that it is excellent not only in corrosion resistance but also in alkali resistance.
If the metal oxide film is one containing the same metal component as the metal component of a metal film which is a primary coat layer (e.g., when an aluminum oxide film (Al2Ox film: 0 less than xxe2x89xa63) is formed on an aluminum film), this film is advantageous in respect of that the adhesion at the interface between the metal film and the metal oxide film is firmer.
Examples of the composite film formed of a plurality of metal oxide components are a Sixe2x80x94Al composite film (SiOx.Al2Oy film: 0 less than xxe2x89xa62 and 0 less than yxe2x89xa63), a Sixe2x80x94Zr composite film (SiOx. ZrOy film: 0 less than xxe2x89xa62 and 0 less than yxe2x89xa62), and a Sixe2x80x94Ti composite film (Siox.TiOy film: 0 less than xxe2x89xa62 and 0 less than yxe2x89xa62). The composite film containing a Si oxide component is advantageous in respect of that the sol solution is relatively stable, and that such film can be formed at a relatively low temperature and hence, the influence to the magnetic characteristic of the magnet can be reduced. The composite film containing a Zr oxide component is advantageous in respect of that it is excellent in alkali resistance.
If the metal oxide film is a composite film containing the same metal component as the metal component of the metal film as the primary coat layer (e.g., when a Sixe2x80x94Al composite oxide film is formed on an aluminum film, or when a Sixe2x80x94Ti composite oxide film is formed on a titanium film), this composite film is advantageous in respect of that the adhesion at the interface between the metal film and the composite film is firmer.
The sol solution used in the sol-gel process is a solution made by preparing a metal compound which is a source for forming a metal oxide film, a catalyst, a stabilizer and water in an organic solvent to produce a colloid by the hydrolytic reaction and the polymerizing reaction, so that the colloid is dispersed in the solution.
Examples of the metal compound as the source for forming the metal oxide film, which may be used, are a metal alkoxide (which may be an alkoxide with at least one alkoxyl group substituted with an alkyl group such as methyl group and ethyl group or with a phenyl group or the like) such as methoxide, ethoxide, propoxide, butoxide; a metal carboxylate such as oxalate, acetate, octylate and stearate; a chelate compound such as metal acetylacetonate; and inorganic salts such as metal nitrate and chloride.
If the stability and cost of the sol solution is taken into consideration, in cases of an aluminum compound used for forming an aluminum oxide film and a zirconium compound used for forming a zirconium oxide film, it is desirable to use an alkoxide having an alkoxyl group containing 3 to 4 carbon atoms such as aluminum and zirconium propoxides and butoxides, a carboxylate such as metal acetate and octylate. In a case of a silicon (Si) compound used for forming a Si oxide film, it is desirable to use an alkoxide having an alkoxyl group containing 1 to 3 carbon atoms such as silicon methoxide, ethoxide and propoxide. In a case of a titanium (Ti) compound used for forming a Ti oxide film, it is desirable to use an alkoxide having an alkoxyl group containing 2 to 4 carbon atoms such as titanium ethoxide, propoxide and butoxide.
To form a composite oxide film, a plurality of metal compounds may be used in the form of a mixture thereof, and a metal composite compound such as a metal composite alkoxide may be used alone or in combination with a metal compound. For example, to form a Sixe2x80x94Al composite oxide film, a Sixe2x80x94Al composite compound such as a Sixe2x80x94Al composite alkoxide having a Sixe2x80x94Oxe2x80x94Al bond and alkoxyl groups (some of which may be substituted with an alkyl group such as methyl group and ethyl group or with a phenyl group or the like) containing 1 to 4 carbon atoms may be used. Particular examples of such compound are (H3CO)3xe2x80x94Sixe2x80x94Oxe2x80x94Alxe2x80x94(OCH3)2 and (H5C2O)3xe2x80x94Sixe2x80x94Oxe2x80x94Alxe2x80x94(OC2H5)2.
When a composite oxide film is to be formed using a plurality of metal compounds, the mixing proportion of each metal compound is particularly not limited, and may be determined in accordance with the proportions of components for a desired composite oxide film.
For example, when a Sixe2x80x94Al composite oxide film is to be formed on an aluminum (Al) film, it is desirable that a Si compound and an Al compound are mixed for use, or a Si compound and a Sixe2x80x94Al composite compound are mixed for use, so that the molar ratio (Al/Si+Al) of aluminum to the total number of moles of silicon (Si) and aluminum (Al) contained in the Sixe2x80x94Al composite oxide film is equal to or larger than 0.001. By mixing such compounds at the above-described molar ratio, the reactivity at the interface with the aluminum film can be enhanced, while maintaining excellent characteristics (the sol solution is stable and the film can be formed at a relative low temperature) in the silicon oxide film. When a heat treatment (which will be described hereinafter) is carried out at 150xc2x0 C. or lower after application of the sol solution to the surface of the metal film, the molar ratio is desirable to be 0.5 or less. When such a treatment is carried out at 100xc2x0 C. or lower, the molar ratio is desirable to be 0.2 or less. This is because it is necessary to rise the temperature in the heat treatment, as the proportion of aluminum mixed is increased.
The proportion of metal compound blended to the sol solution is desirable to be in a range of 0.1% by weight to 20% by weight (in terms of the proportion of the metal oxide, e.g., in terms of the proportion of SiO2 in a case of a Si compound, and in terms of the proportion of SiO2+Al2O3 in a case of a Si compound+an Al compound). If the proportion is lower than 0.1% by weight, there is a possibility that an excessive cycle of the film forming step is required in order to form a film having a satisfactory thickness. If the proportion exceeds 20% by weight, there is a possibility that the viscosity of the sol solution is increased, thereby making it difficult to form the film.
Acids such as acetic acid, nitric acid and hydrochloric acid may be used alone or in a combination as a catalyst. The appropriate amount of acid(s) added is defined by the hydrogen ion concentration in the prepared sol solution, and it is desirable that the acid(s) is added, so that the pH value of the sol solution is in a range of 2 to 5. If the pH value is smaller than 2, or exceeds 5, there is a possibility that the hydrolytic reaction and the polymerizing reaction cannot be controlled at the time of preparing a sol solution suitable for forming a film.
If required, the stabilizer used to stabilize the sol solution may be selected properly depending on the chemical stability of a metal compound used, but a compound capable of forming a chelate with a metal is preferable such as a xcex2-diketone such as acetylacetone, and a xcex2-keto ester such as ethyl acetoacetate.
The amount of stabilizer mixed is desirable to be equal to or smaller than 2 in terms of a molar ratio (stabilizer/metal compound) when the xcex2-diketone is used. If the molar ratio exceeds 2, there is a possibility that the hydrolytic reaction and the polymerizing reaction to prepare the sol solution may be hindered.
Water may be supplied to the sol solution directly or indirectly by a chemical reaction, e.g., by utilizing water produced by an esterifying reaction with a carboxylic acid, when an alcohol is used as a solvent, or by utilizing water vapor in the atmosphere. When water is supplied directly or indirectly to the sol solution, the molar ratio of water/metal compound is desirable to be equal to or smaller than 100. If the molar ratio exceeds 100, there is a possibility that the stability of the sol solution is influenced.
The organic solvent is not limited, and may be any solvent which is capable of homogeneously dissolving all of a metal compound, a catalyst, a stabilizer and water which are components of the sol solution, so that the produced colloid is dispersed homogeneously in the solution. Examples of the organic solvent which may be used are a lower alcohol such as ethanol; a hydrocarbonic ether alcohol such as ethylene glycol mono-alkyl ether; an acetate of hydrocarbonic ether alcohol such as ethylene glycol mono-alkyl ether acetate; an acetate of lower alcohol such as ethyl acetate; and a ketone such as acetone. From the viewpoints of the safety during treatment and the cost, it is desirable that lower alcohols such as ethanol, isopropyl alcohol and butanol are used alone or in combination.
The viscosity of the sol solution depends on the combination of various components contained in the sol solution, and is desirable to be generally equal to or smaller than 20 cP. If the viscosity exceeds 20 cP, there is a possibility that it is difficult to form a film uniformly, and cracks may be generated during a thermal treatment.
The time and temperature for preparing the sol solution depend on the combination of various components contained in the sol solution. Usually, the preparing time is in a range of 1 minute to 72 hours, and the preparing temperature is in a range of 0xc2x0 C. to 100xc2x0 C.
Examples of the method for applying the sol solution to the surface of the metal film, which may be used, are a dip coating process, a spraying process and a spin coating process.
After application of the sol solution to the surface of the metal film, the applied sol solution is subjected to a heat treatment. The heating temperature required may be a level enough to evaporate at least the organic solvent. For example, when the ethanol is used as the organic solvent, the minimum temperature is 80xc2x0 C. which is a boiling point of the ethanol. On the other hand, when a sintered magnet is used, if the heating temperature exceeds 500xc2x0 C., there is a possibility that the degradation of the magnetic characteristic of the magnet is caused, or the metal film is molten. Therefore, the heating temperature is desirable to be in a range of 80xc2x0 C. to 500xc2x0 C., and more preferably, is in a range of 80xc2x0 C. to 250xc2x0 C. from the viewpoint for preventing the generation of cracks during cooling after the heat treatment to the utmost. When a bonded magnet is used, the temperature condition for the heat treatment must be set in consideration of the heat-resistant temperature of a resin used. For example, when a bonded magnet made using an epoxy resin or a polyamide resin is used, the heating temperature is desirable to be in a range of 80xc2x0 C. to 200xc2x0 C. in consideration of the heat-resistant temperatures of these resins. Usually, the heating time is in a range of 1 minute to 1 hour.
According to the above-described process, a metal oxide film essentially comprising an amorphous phase, which is excellent in corrosion resistance, can be formed. For example, with a Sixe2x80x94Al composite oxide film, the structure thereof includes a large number of Sixe2x80x94Oxe2x80x94Si bonds and a large number of Sixe2x80x94Oxe2x80x94Al bonds, when in a case of a Si-rich film, and includes a large number of Alxe2x80x94Oxe2x80x94Al bonds and a large number of Sixe2x80x94Oxe2x80x94Al bonds in a case of an Al-rich film. The proportions of both the components in the film are determined by a proportion of metal compound mixed.
According to the above-described process, the metal oxide film contains carbon (C) due to the metal compound and the stabilizer. The metal oxide film essentially comprising an amorphous phase, which is excellent in corrosion resistance, is produced easily by the containment of carbon, and it is desirable that the carbon (C) content is in a range of 50 ppm to 1,000 ppm (wt/wt) . If the C content is smaller than 50 ppm, there is a possibility that cracks are generated in the film. If the C content exceeds 1,000 ppm, there is a possibility that the densification of the film does not occur sufficiently.
The metal oxide film formed by the above-described process has a thickness set in a range of 0.01 xcexcm to 1 xcexcm, because if the thickness is smaller than 0.01 xcexcm, there is a possibility that an excellent corrosion resistance cannot be exhibited under given conditions, and if the thickness exceeds 1 xcexcm, there is a possibility that cracks are generated in the film or the peeling-off of the film occurs due to a variation in temperature, and thus, an excellent thermal shock resistance cannot be exhibited. To exhibit an excellent corrosion resistance under given conditions and an excellent thermal shock resistance to a variation in temperature, it is desirable that the thickness of the metal oxide film is in a range of 0.05 xcexcm to 0.5 xcexcm. Of course, if required, the application of the sol solution to the surface of the metal film and the subsequent heat treatment may be conducted repeatedly a plurality of times.
A shot peening (a process for modifying the surface by bumping hard particles against the surface) may be carried out as a previous step before the formation of the metal oxide film on the metal film. The metal film can be smoothened by carrying out the shot peening, thereby facilitating the formation of a metal oxide film which is thin, but has an excellent corrosion resistance.
It is desirable that a powder having a hardness equivalent to or more than the hardness of the formed metal film is used for the shot peening. Examples of such powder are spherical hard particles having a Mohs hardness of 3 or more such as steel balls and glass beads. If the average particle size of the powder is smaller than 30 xcexcm, the pushing force applied to the metal film is smaller and hence, a lot of time is required for the treatment. On the other hand, if the average particle size of the powder exceeds 3,000 xcexcm, there is a possibility that the smoothness of the surface is too large, and the finished surface is uneven. Therefore, the average particle size of the powder is desirably in a range of 30 xcexcm to 3,000 xcexcm, and more desirably in a range of 40 xcexcm to 2,000 xcexcm.
The blast pressure in the shot peening is desirable to be in a range of 1.0 kg/cm2 to 5.0 kg/cm2. If the blast pressure is lower than 1.0 kg/cm2, there is a possibility that the pushing force applied to the metal film is smaller and a lot of time is required for the treatment. If the blast pressure exceeds 5.0 kg/cm2, there is a possibility that the pushing force applied to the metal film is ununiform, thereby bringing about the degradation of the smoothness of the surface.
The blast time in the shot peening is desirable to be in a range of 1 minute to 1 hour. If the blast time is shorter than 1 minute, there is a possibility that the uniform treatment of the entire surface cannot be achieved. If the blast time exceeds 1 hour, there is a possibility that the degradation of the smoothness of the surface is brought about.
A rare earth element (R) contained in an Fexe2x80x94Bxe2x80x94R based permanent magnet used in the present invention is desirably at least one element from among Nd, Pr, Dy, Ho, Tb and Sm, in addition thereto at least one element from among La, Ce, Gd, Er, Eu, Tm, Yb, Lu and Y.
Usually, one of them (R) suffices, but in practice, a mixture of two or more rare earth elements (misch metal and didymium and the like) may be used for the reason of a procurement convenience.
The content of R in an Fexe2x80x94Bxe2x80x94R based permanent magnet is desirable to be in a range of 10% by atom to 30% by atom. If the R content is lower than 10% by atom, the crystal structure is the same cubic crystal structure as xcex1-Fe and for this reason, a high magnetic characteristic, particularly, a high coercive force (iHc) is not obtained. On the other hand, if the R content exceeds 30% by atom, the content of an R-rich non-magnetic phase is increased, and the residual magnetic flux density (Br) is reduced, whereby a permanent magnet having an excellent characteristic is not produced.
The Fe content is desirable to be in a range of 65% by atom to 80% by atom. If the Fe content is lower than 65% by atom, the residual magnetic flux density (Br) is reduced. If the Fe content is exceeds 80% by atom, a high coercive force (iHc) is not obtained.
It is possible to improve the temperature characteristic without degradation of the magnetic characteristic of the produced magnet by substituting a portion of Fe with Co. However, if the amount of Co substituted exceeds 20% of Fe, the magnetic characteristic is degraded and hence, such amount is not preferred. The amount of Co substituted in a range of 5% by atom to 15% by atom is desirable for providing a high magnetic flux density, because the residual magnetic flux density (Br) is increased, as compared with a case where a portion of Fe is not substituted.
The B content is desirable to be in a range of 2% by atom to 28% by atom. If the B content is smaller than 2% by atom, a rhombohedral structure is a main phase, and a high coercive force (iHc) is not obtained. If the B content exceeds 28% by atom, the content of a B-rich non-magnetic phase is increased, and residual magnetic flux density (Br) is reduced, whereby a permanent magnet having an excellent characteristic is not produced.
To improve the manufacture of the magnet and reduce the cost, at least one of 2.0% by weight of P and 2.0% by weight of S may be contained in a total amount of 2.0% by weight or less in the magnet. Further, the corrosion resistance of the magnet can be improved by substituting a portion of B with 30% by weight or less of carbon (C).
Further, the addition of at least one of Al, Ti, V, Cr, Mn, Bi, Nb, Ta, Mo, W, Sb, Ge, Sn, Zr, Ni, Si, Zn, Hf and Ga is effective for improving the coercive force and the rectangularity of a demagnetizing curve and for improving the manufacture and reducing the cost. It is desirable that at least one of them is added in an amount within a range satisfying a condition that at least 9 kG of Br is required in order to ensure that the maximum energy product (BH)max is equal to or larger than 20 MGOe.
In addition to R, Fe and B, the Fexe2x80x94Bxe2x80x94R based permanent magnet may contain impurities inevitable for industrial production of the magnet.
The Fexe2x80x94Bxe2x80x94R based permanent magnet used in the present invention has a feature in that it includes a main phase comprising a compound having a tetragonal crystal structure with an average crystal grain size in a range of 1 xcexcm to 80 xcexcm, and 1% to 50% by volume of a non-magnetic phase (excluding an oxide phase). The magnet shows iHcxe2x89xa71 kOe, Br greater than 4 kG and (BH) maxxe2x89xa710 MGOe, wherein the maximum value of (BH)max reaches 25 MGOe or more.
A further film may be formed on the metal oxide film of the present invention. By employing such a configuration, it is possible to enhance the characteristic of the metal oxide film and provide a further functionability to the metal oxide film.